A right circular cylinder has a volume of (6+23)π cm3(6 + 2\sqrt{3})\pi\text{ cm}^3 and a base radius of (1+3) cm(1 + \sqrt{3})\text{ cm}. Find, without using a calculator, the height of the cylinder, in cm, in the form (a+b3)(a + b\sqrt{3}), where aa and bb are integers.

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Worked solution

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1. Set up the equation for the volume of the cylinder

Let the height of the cylinder be hh.
Volume V=πr2hV = \pi r^2 h
(6+23)π=π(1+3)2h(6 + 2\sqrt{3})\pi = \pi (1 + \sqrt{3})^2 h

2. Expand the square of the radius and simplify the equation

(6+23)=(1+23+3)h(6 + 2\sqrt{3}) = (1 + 2\sqrt{3} + 3) h
6+23=(4+23)h6 + 2\sqrt{3} = (4 + 2\sqrt{3}) h
h=6+234+23h = \frac{6 + 2\sqrt{3}}{4 + 2\sqrt{3}}
h=3+32+3h = \frac{3 + \sqrt{3}}{2 + \sqrt{3}}

3. Rationalise the denominator to find the height in the required form

h=3+32+3×2−32−3h = \frac{3 + \sqrt{3}}{2 + \sqrt{3}} \times \frac{2 - \sqrt{3}}{2 - \sqrt{3}}
h=6−33+23−34−3h = \frac{6 - 3\sqrt{3} + 2\sqrt{3} - 3}{4 - 3}
h=3−31h = \frac{3 - \sqrt{3}}{1}
h=3−3h = 3 - \sqrt{3}

Answer

2−32 - \sqrt{3}

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